I've never actually paid attention to that. I don't use the Activity Stream quite that way. I did have a look, though, and I don't even have a guess as to what it might be. It seems pretty random as to which avatars show and which don't. Very strange!

It's probably some setting in the back end, but I have absolutely no clue where or what that would be. I'll have to think on it, and make have a look and see what I can find. The board software is... quirky.